Discovery subnets by available ip addresses

[oliviassss]

Issue

#2056

Description

Add a new step in subnet discovery, which is to select subnets based on the count of available IP addresses.

• ALB：To ensure that the load balancer can scale properly, each Availability Zone subnet has at least 8 free IP addresses per subnet. (Ref: https://docs.aws.amazon.com/elasticloadbalancing/latest/application/application-load-balancers.html#subnets-load-balancer)

• NLB: For internet-facing load balancers, the subnets must have at least 8 available IP addresses. For internal load balancers, this is only required if the user let AWS select a private IPv4 address from the subnet. (Ref: https://docs.aws.amazon.com/elasticloadbalancing/latest/network/network-load-balancers.html)

Checklist

• Added tests that cover your change (if possible)
• Added/modified documentation as required (such as the README.md, or the docs directory)
• Manually tested
• Made sure the title of the PR is a good description that can go into the release notes

BONUS POINTS checklist: complete for good vibes and maybe prizes?! 🤯

• Backfilled missing tests for code in same general area 🎉
• Refactored something and made the world a better place 🌟

[k8s-ci-robot]

Hi @oliviassss. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[codecov-commenter]

Codecov Report

Merging #2146 (857ce64) into main (8f1c32c) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #2146      +/-   ##
==========================================
+ Coverage   52.04%   52.11%   +0.06%     
==========================================
  Files         132      132              
  Lines        7130     7148      +18     
==========================================
+ Hits         3711     3725      +14     
- Misses       3131     3135       +4     
  Partials      288      288              

Impacted Files	Coverage Δ
pkg/ingress/model_build_load_balancer.go	51.08% <100.00%> (+0.21%)	⬆️
pkg/networking/subnet_resolver.go	92.52% <100.00%> (+0.36%)	⬆️
pkg/service/model_build_load_balancer.go	83.17% <100.00%> (-1.22%)	⬇️
pkg/service/model_build_listener.go	91.20% <0.00%> (ø)

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[kishorj]

/ok-to-test

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: kishorj, oliviassss

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[kishorj]

/lgtm

[mikestef9]

Can we add a doc update for this feature? Probably here https://kubernetes-sigs.github.io/aws-load-balancer-controller/v2.3/deploy/subnet_discovery/

[oliviassss]

@mikestef9 Sure, will update it in the patch release, thanks.